Positive feed power drills have a motor for turning a spindle through a drive gear train. For advancing the spindle as a bit drills through a workpiece, the spindle is threaded into a feed gear which turns at a predetermined rate faster than the turning rate of the drill bit. The feed gear is driven by a gear train usually from the same motor as the drive gear train. The gear ratio of the drive gear train is selected to be slightly greater than the gear ratio of the feed gear train so the feed gear will turn a fraction of an rpm faster than the drive gear. In that way, the drill bit and spindle are advanced a predetermined amount for each turn of the drill bit and spindle. Once the spindle and drill bit have been advanced a sufficient distance, a mechanism is usually actuated to disengage the feed gear train from the motor and lock it in place so that, as the motor continues to turn the spindle drive gear in the same direction, the spindle turns inside the locked feed gear to rapidly retract the spindle and drill bit from the workpiece.
Many companies which manufacture aircraft are presently utilizing overlay materials of different types and hardness. For weight reduction (with minimum loss of strength) and stealth characteristics, graphite and kevlar composites are laid over titanium, stainless steel, etc. It has been found desirable to drill through the composite overlay material at high rpm (revolutions per minute) and to drill through the harder base metal at a lower rpm.
There has been provided in prior art, a two-speed positive-feed drill which can be programmed to change speeds back and forth in response to the depth the tool has drilled. This tool requires that an air signal be sent to a cylinder to shift clutches in the gear train to bring planetary gear reductions in and out of the system as the drill bit reaches predetermined depths in the workpiece.
Because of the complex curvatures of portions of modern aircraft, the thickness of the material is in constant variance. Because of the constant variance of material thickness, the usefulness of a tool that would change speed as a result of drill depth would be greatly limited. To enhance the usefulness of the prior art tool, the programmable depth sensing features have been replaced with electronic load cells and valving that supplies air to the speed changing cylinder as a result of the thrust generated by the drill bit. Unfortunately, the prior art tool is now very large, expensive and is really not suited for production drilling.